Stigma Volume XII No.4, Oktober – Desember 2004
EFFECTS OF LAND USE CHANGE ON SOIL ORGANIC MATTER STATUS
OF BULK AND FRACTIONATED SOIL AGGREGATES
(Pengaruh perubahan penggunaan lahan terhadap status bahan organik aggregat tanah alami dan aggregat yang difraksionasi)
Yulnafatmawita*)
ABSTRAK
Perubahan penggunaan lahan dari ekosistem alami menjadi lahan pertanian akan mempengaruhi kondisi fisik suatu tanah. Hal ini sehubungan dengan perubahan kandungan bahan organik tanah, salah satu agen pengikat dan pemantap struktur tanah. Perbedaan intensitas pengolahan tanah terhadap suatu areal dalam mempersiapkannya menjadi lahan pertanian akan menyebabkan kehilangan bahan organik yang berbeda serta derajat kehancuran aggregat yang berbeda pula. Penelitian ini bertujuan menganalisis status C-organik dan distribusinya dalam aggregat dari 2 jenis tanah akibat perubahan penggunaan lahan. Sampel tanah yaitu Oxisol (Ferrosol=ASC) didominasi oleh liat kaolinit dari Lamington dan Vertisol (Vertosol=ASC) didominasi oleh liat montmorillonit dari Goondiwindi, Queensland Australia. Kandungan C-organik, N total, C/N ratio dianalisis dari aggregat tanah alami dan aggregat yang difraksionasi. Perubahan penggunaan lahan dari hutan hujan tropis menjadi padang rumput pada Oxisol menurunkan C-organik tanah dari 6.9 menjadi 5.6%, N total dari 0.63 menjadi 0.53% tetapi tidak nyata mempengaruhi C/N rationya. Sedangkan pada tanah Vertisol, C-organik menurun dari 2.0 menjadi 1.2%, N total dari 0.21 menjadi 0.15%. Akan tetapi perubahan penggunaan lahan untuk kedua tanah tidak merubah pola distribusi kandungan C-organik dan N total tanah.
Key words: land use change, soil organic carbon (SOC), fractionated soil aggregate, energy input, degradation
INTRODUCTION
As indicated by organic carbon (OC), soil organic matter (SOM) plays an important role in agricultural soil and environmental sustainability. Its accumulation and interaction with soil mineral materials will improve soil structural stability as well as the environmental quality and will reduce run-off and erosion, as well as CO2 release causing global warming. It means that if soil carbon could be maintained above the critical level, we can avoid soil and environmental damage which can spend a lot of money to rehabilitate them. Therefore, soil organic carbon can be considered as “a black
gold” in agricultural environment if it is well-managed.
Total C in soils is the sum of both organic and inorganic C. Soil organic C (SOC) is present at all agricultural soils (Nelson and Sommers, 1996). Actually, total amount of SOC in the world's soils contains about three times as much carbon as it is found in the world's vegetation (Brady and Weil, 1999). However, as the OC is utilized by decomposer organisms, it will be either assimilated into their body tissues and then released as metabolic products, or respired as CO2 which can cause global warming at high emission rate.
Factors determining SOC levels can be classified into 5 parts: 1) climate, 2) soil mineral parent materials and product of pedogenesis. 3) Biota: vegetation and soil organisms, 4) topography, and 5) land management practices (Baldock and Nelson, 2000). Land management is the only factor which is affected by human action. Land use change, for example conversion of native pasture to farming land (either under conservation or conventional tillage) has been reported to decrease SOC content of the soils (Yulnafatmawita, 2004), and increase clay dispersion of tropical soil and water stable microaggregates < 0.25 mm (Mbagwu and Picollo, 1998). As reported by Skjemstad et al. (1986) that continuous cultivation for 20, 35, and 45 years was resulted in 50, 61, and 66% loss in SOM, respectively, from the 0- to 0.10 m soil layer. Organic C contained in the soil organic fraction consists of various stages of decomposition, stable "humus" synthesized from residues, and highly carbonized com-pounds such as charcoal, graphite and coal (elemental forms of C) (Nelson and Sommers, 1996). The difference will result in different rate of SOM oxidation or decomposition. Additionally, the rate of SOM depletion is also affected by type of land use or agricultural system adopted.
*) Fakultas Pertanian Universitas Andalas Padang
ISSN 0853-3776 AKREDITASI DIKTI No. 52/DIKTI/KEP/1999 tgl. 12 Nopember 2002
Based on the above information it can be concluded, that SOC is affected by land use management that is induced by human action, besides types of SOM in the soils, environmental factors and the soil characteristics itself. Therefore, this experiment was conducted to identify carbon and nitrogen content of soils from different land use and soil clay mineralogy, and the distribution of both elements within the soil aggregates in temperate region.
METHODOLOGY
Two types of soil Vertisol and Oxisol used in this experiment were collected from different location (Goondiwindi and Lamington, Qld Australia), and different land use, Brigalow forest (CF) and annual cultivation (CC) for Vertisol; rainforest (ORF) and pasture (ORP) for Oxisol. Soils were sampled from 0-10 cm depth after removing all of fresh OM on the surface. The samples were then air-dried and ground to pass 2-mm sieve. Some of the sample was fractionated using 3 energy level (40, 60, and 200 J g-1 soil) and 3 aggregate sizes (> 20, 20-2, and < 2 um) to imitate the hierarchical nature of the aggregates themselves.
All of soil samples being fractionated (CC, CF, ORP, ORF) and the bulk soils were analyzed for the organic carbon (OC), total carbon (TC), and nitrogen (N). Modification of Walkley & Black method was employed to analyze soil OC content, and the LECO method was used to determine TC and N content.
Total Carbon(TC) and Nitrogen (N) using LECO method
Approximately 0.5-0.8 g AD-soil (< 0.5 mm) of each fraction and bulk soil was weighed into a ceramic boat, and directly combusted within high temperature LECO CNS-2000. In this experiment, temperature employed was 1100oC, since the CO3 concentration of the soils was quite low.
Organic Carbon (OC) using Walkley and Black method
This method was modified for the heating process. The soil samples, after the addition of potassium dichromate (K2Cr2O7.2H2O) and sulphuric acid H2SO4, were heated on a hot plate at 120oC for 30 minutes for homogenizing the heating process. Other than this additional heating process, the procedure conducted was similar to that for the original Walkley and Black method.
Additionally, absolute OC content in each soil fraction was determined by multiplication of the OC concentration with the soil weight in each fraction, and then divided by the total soil weight. This is aimed to find out where the SOC (SOM) was mostly accumulated within soil aggregates. Finally, the value of C to N (C/N) ratio was calculated to predict the susceptibility of the SOC to biological degradation for all soil samples.
RESULTS AND DISCUSSION
1. Soil Carbon
Bulk Soil. Organic carbon and total carbon content (Table 1) of the bulk soils were linearly correlated (%OC=0.85*[%TC]; R=0.99) and therefore the explanation for OC will bethe same as for TC.
Table 1 Organic carbon (OC) and total carbon (TC) of fractionated and bulk soil samples Soil Sample Soil Fractions Bulk Soil* Weighted Mean*
> 20 m 2-20 m <2 m (BS) (WM) ---- % ---OC
CC 0.7(0.2)** 2.9(0.7) *** 1.4(0.2)*** 1.2 1.2
CF 1.4(0.1) 4.1(1.8) 2.3(0.5) 2.0 1.9
ORP 6.7(0.4) 7.2(0.6) 4.1(0.5) 5.6 5.3
ORF 8.6(0.7) 8.3(1.0) 4.5(0.9) 6.9 7.2
TC
CC 0.8(0.3) 3.1(0.7) 1.6(0.2) 1.4 1.3
CF 1.7(0.2) 4.6(1.6) 2.6(0.5) 2.6 2.3
ORP 7.1(0.5) 7.5(0.8) 4.5(0.5) 6.3 6.5
ORF 8.6(0.5) 8.0(0.8) 4.6(0.7) 8.2 7.2
_____________________________________________________________________________Note: *The OC correlation is very good with R²=0.99 --- %WM = 1.01 * [%BS]
*The TC correlation is very good with R2=0.98--- %WM = 0.93 * [%BS]
**The values in brackets are S.D. with n=6 ***The values in brackets are S.D. with n=12 The concentration of both OC and TC in Oxisol (ORF and ORP) were significantly (P<0.01) higher (>5%) than in Vertisol (CF and CC). This
higher annual rainfall (±1900 mm) than the Vertisol (±500 mm) did. High precipitation in the Oxisol had provided enough soil water storage for plant growth throughout the year. As a result, litter, dead roots, microbial products accumulated with time and built up the SOM, as the soil was not physically disturbed by any degradation process.
Land use change within the same soil type had also changed the SOC content. It was not only caused by the difference in the SOM source, but also by the change in the turnover rate of the SOM due to the change in the environmental condition. The SOC value of the rain forest (6.9%) was higher than that at the pasture site (5.6%). It shows that the change of land use from rainforest to pasture for about 100 years (based on land owner discussion) decreased SOC content of Oxisol by 1.3 % (≈19% of the rainforest SOC). The depletion of SOM by cultivation was also noticed in Vertisol soils (CC and CF). Annual cultivation for monoculture grain crop (CC) had decreased the SOM level of Vertisol from the original Brigalow forest (CF) in Goondiwindi, indicated by the reduction of the SOC concentration of the bulk soil by 0.8% (≈ 40% of the Brigalow forest SOC). The decrease of SOC content from natural forest (67 g kg-1) to traditional farming (24 g kg-1) at 0-15 cm soil depth in Kolombangara, Solomon Islands was also reported by Wairiu and Lal (2003). Continuous cultivation for 20, 35, and 45 years resulted in 50, 61, and 66% loss in organic matter, respectively, from the 0- to 0.1m layer (Skjemstad
et al.,1986). Soil OC reduction was also reported by Watts et al. (2000) for about 2.1% from permanent grassland (3.2%) to permanent fallow (1.1 %) from the same soil and the same environmental condition.
Fractionated soil. Table 1 also presents the data of OC and TC contents of fractionated soils. Each soil type had different concentration and showed specific pattern of OC and TC content. However, they had strong linear correlation between the weighted mean of the aggregate fractions and the bulk soils. The slopes of the regressions were very close to 1 (1.05 and 0.94) with a high coefficient of correlation (R =0.99) for both OC and TC. These indicated that the fractionation procedure did not cause significant material lost, especially the soil organic matter content. Since the patterns are similar for both OC and TC content, only the OC curve is presented (Figure 1) as a function of mean aggregate size for both soils, Oxisol and Vertisol.
In the Vertisol, the OC content of fraction 2-20 m was significantly (P <0.01) higher than those of the other 2 fractions. Higher OC
concentration in the 2-20 m fraction of this soil was associated with the nature of the smectitic clay soils. When the soil swells at high water content, some of the aggregates as well as the binding SOM will break into smaller parts. The SOM will associate with the smaller aggregates, therefore, as the soil water content decreases, the SOM will stay and be occluded within the smaller aggregates. The SOM which was left on the surface of the larger aggregates (> 20 m) or some of the free particulate ones will be decomposed and depleted faster than that incorporated in the smaller aggregates. As a consequence, the largest aggregate (> 20 m) fraction had significantly lower OC content than the < 2 and 2-20 m fractions.
Figure 4.4.1 Organic carbon of fractionated Oxisol in rain-forested (ORF) and pastured (ORP) land use as well as fractionated Vertisol in cultivated (CC) and Brigalow forested (CF) land use as a function of mean aggregate size
easily attacked by the decomposing microbes, especially during cultivation. As found by Mbagwu and Piccolo (1998), SOM in macro-aggregates was not strongly adsorbed on the particles unlike the strong adsorption on clay and silt size particles.
On the other hand, OC concentration in the 2-20 m fraction was the highest among the fraction size and also above the OC content of the bulk soil. This is due to close association between particles building this fraction with SOM. Fraction 2-20 m must be arranged from the smaller size particles, such as clay particles which can create strong complexation with the bonding agent. This is in agreement with the results reported by Conteh and Blair (1998) that OM presents in the cracking clay soils they used for cotton production was mostly concentrated in the micro-aggregates of the soil. Even though the absolute OC content of fractionated Vertisol in both land use (CC and CF) was not significantly different among all fractions (Figure 1).
In contrast to Vertisol, Oxisol had higher OC content in the larger (> 20 and 2-20 µm) fractions, and they significantly decreased as the fraction size decreased to < 2 m. While the absolute OC content significantly decreased as aggregate size decreased from the > 20 m to the <2 m aggregate fraction. Actually, as stated above that finer soil particles will retain more OM due to their larger surface area, on which OM can be attached to through several mechanisms. Therefore, lowest OC concentration in the
smallest fraction indicates that the < 2 m fractions of Oxisols O’Reillys seemed to have reached their maximum capacity to retain or to make complexation with OM, which was also less than the capacity of the bulk soil to retain OM. This is found to be true since kaolinite dominating this soil has much lower CEC (1-10 meq/100 g) and surface area (15-20 m2/g) than smectitic clay mineral (80-120 meq/100 g and 280-500 m2g-1 respectively for CEC and surface area) on where SOM can be attached (McBride, 1994; Filep, 1999).
In the Oxisol, we could expect a continuous high rate of particulate organic matter (POM) addition in the larger aggregates. Besides what has been explained above, smallest fraction reached its maximum capacity in complexing SOM, this was also due to that the soil was never cultivated but was pastured and rain-forested for a long time. The growth cycle of the pasture and rain forest would accumulate SOM by time. Thus, maintaining a high SOM content in the larger fractions.
However, the SOM in the larger fraction was not strongly bound to the soil matrix as in < 2
m fraction. That is why SOC significantly decreased in the > 20 m fraction as the land use was changed from rain forest to pasture.
2. Total Nitrogen
Table 2 presents total nitrogen (TN) of bulk and fractionated soil samples. Total nitrogen of fractionated soils (Vertisol and Oxisol) tended to follow the OC and the TC content
Table 2. Nitrogen content of soil samples using LECO Method
Soil Sample Soil Fractions
> 20 m 2-20 m <2 m Bulk Soil* Wt.Mean* %
---CF 0.09(0.06) 0.37(0.07) 0.17(0.05) 0.21 0.14
CC 0.06(0.02) 0.24(0.06) 0.16(0.03) 0.15 0.11
ORF 0.65(0.03) 0.61(0.04) 0.40(0.08) 0.63 0.56
ORP 0.56(0.02) 0.60(0.06) 0.47(0.05) 0.53 0.54
Note: * The N correlation is good with R²=0.96 --- %WM = 0.92 * [%BS] ** The values in brackets are S.D. with n = 6
Nitrogen concentration 0.06% as the land use changed from Brigalow forest (CF) to annual cultivation for grain crop (CC), as well as by ± 0.10% follow the C content in each bulk and the pattern and in the values, except for CF soil. The pattern tended to decrease by decreasing aggregate size fraction from > 20 to < 2 µm. C/N ratio brackets are S.D. with n=6
** The values in that the SOM had been cycled through on the C/N ratios, the SOM in Brigalow
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